The invention relates to the field of semiconductor devices, and in particular to a lateral thyristor structure for protection against electrostatic discharge.
FIG. 1 illustrates a block diagram of a circuit arrangement for protection against electrostatic discharge. The circuit arrangement includes at least one interior circuit 4 that requires protection, two supply terminals 1, 2, and one terminal 3 for input or output. In the event of an electrostatic discharge (ESD) in the input/output, a damaging voltage/current can reach the functional elements of the interior circuit 4. The discharge of damaging voltage/current can also reach the supply lines 1, 2 through bipolar parasitic elements, which are always present. In both cases irreversible damage can be done to the integrated circuit. Consequently, clamping devices 5 are typically attached to the supply connections 1, 2, to protect the supply lines and the elements associated therewith. Integrated circuits generally have a certain amount of self-protection through inherent parasitic bipolar elements, but this is generally insufficient protection.
In the case of circuits that operate with high supply voltages (e.g., 8 V), and thus have special high-voltage transistors with a drift zone, the inherent parasitic elements often trigger too late or not at all in order to adequately protect the circuit. Therefore, as a practical matter self-protection does not exist and the input/output 3 can be damaged by ESD.
German initial Patent Disclosure DE 41 35 522 discloses adding a clamping circuit 6 to the input/output for clamping against a supply line 2. In the event of an electrostatic discharge (ESD), this reduces the voltage at the input/output to a tolerable value and arrests the discharge. As a rule, a clamping circuit also protects the other supply connection 1, for example with a diode 7. For most input/output circuits, only a parallel protector is feasible, since as a rule a series resistor must not be inserted. Thyristors, field oxide transistors, or diodes are often used as protection elements.
A problem with diodes is that the voltage at the diode can become quite large in the case of high currents, especially in the non-conducting direction. As a result, sensitive gate oxides and diffusions in the interior circuit that is being protected may be damaged. Furthermore, diodes are rather cumbersome due to their large area, especially in the case of integrated circuits with several inputs and/or outputs.
Field oxide transistors can clamp the voltage. However, depending on the external or internal circuitry of the input/output, after firing the field oxide transistor in the application circuit may remain in a low-ohm state with average voltage, thus interfering with the function of the input/output. For example, if an external source can deliver sufficient current at high voltage, it can happen that a transient at the input/output triggers the clamping element and it is no longer quenched, due to the soft firing characteristic of the field oxide transistor structure. However, this disturbs the function of the circuit to such an extent that only turning it off and then on again quenches the field oxide transistor structure and reestablishes the original functionality of the circuit.
Transistors without any special firing arrangement draw so much current in their switched-on state (“hard firing characteristic”) that the external-internal source is strongly loaded and the thyristor quenches due to the voltage collapse of the source. However, the protective effect is limited by a relatively high firing voltage so it is possible for the circuit to be damaged before the firing voltage is reached.
A special high-voltage thyristor that includes a high-voltage transistor as the firing element clamps the voltage to levels low enough that no sensible external/internal circuitry can cause a “stuck condition”. However, the high-voltage transistor fires so late due to the drift zone that sensitive structures may be damaged. For example, FIG. 2 illustrates a cross sectional view of a prior art lateral thyristor structure for protection against electrostatic discharge. An example of such a prior art device is disclosed in the paper entitled “A Process-Tolerant Input Protection Circuit for Advanced CMOS Processes” by R. N. Rountree, EOS/ESD Symposium Proceedings, 1988, pages 201-205. The lateral thyristor structure is integrated into a weakly p-doped semiconductor substrate 10. The weakly p-doped semiconductor substrate 10 is electrically connected to a cathode 16, via the substrate contact 19 that is a strongly p-doped region. A weakly n-doped well region 11 is diffused into the weakly p-doped semiconductor substrate 10.
A first strongly n-doped region 12 is diffused into the surface of the weakly p-doped semiconductor substrate 10. This n-doped region is also connected to the cathode 16.
A strongly p-doped region 13 and a strongly n-doped region 14 are diffused into the surface of the weakly n-doped well region 11. The strongly p-doped region 13 and the strongly n-doped region 14 are both electrically connected to an anode 17.
The strongly p-doped region 13 and the strongly n-doped region 14 adjoin one another. A field oxide region 15 is situated between the first strongly n-doped region 12 and the strongly p-doped region 13. The field oxide region 15 is situated spatially above the pn junction which is formed between the weakly n-doped well region 11 and the weakly p-doped semiconductor substrate 10. Finally, the lateral thyristor structure is spatially separated from other semiconductor structures, which are not shown here, which are introduced into the semiconductor substrate 10, and which form the circuit that is being protected.
The strongly n-doped region 12 forms the emitter of a pnp transistor; the weakly p-doped semiconductor substrate 10 forms the base of the npn transistor; and the weakly n-doped well region 11 forms the collector of the npn transistor. The weakly p-doped semiconductor substrate 10 furthermore forms the collector of a pnp transistor. The weakly n-doped well region again forms the base for this pnp transistor. Finally, the strongly p-doped region 13 forms the emitter of this pnp transistor.
When the voltage between the anode 17 and the cathode 16 is less than about 50 volts, the pn junction between the weakly n-doped well region 11 and the weakly p-doped semiconductor substrate 10 is nonconducting, so that the lateral thyristor structure conducts only a very small current. However, if the voltage between the anode 17 and the cathode 16 exceeds approximately 50 volts, an avalanche breakdown occurs so charge carriers are generated at the pn junction between the weakly n-doped well region 11 and the weakly p-doped semiconductor substrate 10. This avalanche breakdown generates charge carriers at the base-collector junctions of the two bipolar transistors, namely the npn transistor and the pnp transistor, so the lateral thyristor fires.
Therefore, there is a need for a thyristor structure to protect against electrostatic discharge, which has a low firing voltage and does not remain stuck in its switched-on state.